Fix another incorrect message for WTAP_ERR_UNSUPPORTED.

WTAP_ERR_FILE_UNKNOWN_FORMAT is reported if the file is in a format that
libwiretap doesn't know about (either because it's not a capture file at
all or because it's a capture file in a format it doesn't support).
WTAP_ERR_UNSUPPORTED is for files in a *known* format that are using
features or file format elements (record type, link-layer header type,
etc.) that libwireshark doesn't support.  Fix another copy-and-pasteo
causing WTAP_ERR_UNSUPPORTED to be reported with a message appropriate
for WTAP_ERR_FILE_UNKNOWN_FORMAT.

Change-Id: Ib35e2a6b6c62517887e0386f0fed1795db335439
Reviewed-on: https://code.wireshark.org/review/5810
Reviewed-by: Guy Harris <guy@alum.mit.edu>